This invention relates generally to salt splitting methods, and more specifically, to improved methods for salt splitting by means of bipolar membrane electrodialysis where the salts possess cations having the potential of forming substantially insoluble precipitates, e.g., metal hydroxides, oxides, and so on.
Salt splitting electrodialysis is a useful technique for recovering acid and base values from salt streams.
Electrodialysis methods performed with bipolar membranes have been used where the cation is monovalent and does not form an insoluble base (, e.g., lithium, sodium, potassium and ammonium). Salt splitting methods using electrodialysis with bipolar membranes traditionally have not been carried out in the presence of polyvalent metal salts because metal hydroxides formed are so insoluble they precipitate in the electrodialysis chamber, or in or on the cation or bipolar membranes, fouling the membranes or electrodialysis stacks resulting in blockage and eventual shutdown. The formation of insoluble metal hydroxides within the cation or bipolar membranes will also lead to higher voltage drops, and eventually physical damage to the membranes. One report published by Aqualytics (Salem, Eli et al., xe2x80x9cBipolar Membrane Water Splitting to Produce Organic Acidsxe2x80x9d, presented at The 11th. International Forum on Electrolysis in the Chemical Industry, Clearwater Beach, FL (Nov. 2-6, 1997), a manufacturer of membranes and electrodialysis equipment with bipolar membranes, suggests treatment with chelating ion exchange resins to remove polyvalent metals to  less than 1 ppm before employing electrodialysis with bipolar membranes.
This limitation in the use of salt splitting electrodialysis with bipolar membranes has been found to be particularly problematic in the production of ascorbic acid intermediates by glucose fermentation. In this process, fermentation converts glucose to a 2-keto-L-gluconic (also referred to as xe2x80x9cKLGxe2x80x9d) acid salt, a useful intermediate in the production of ascorbic acid. KLG acid salt, however, must first be converted to the free acid form before chemical conversion to ascorbic acid can occur. The problem is, that free KLG acid is a fairly strong organic acid (pKa=2.7). Solutions of KLG acid were found to be too acidic for fermentation to proceed. Consequently, a suitable base needs to be added to the fermenter to maintain a near neutral pH, and form a salt of KLG. In the case of bacterium used to form KLG from glucose, the most suitable base has been divalent calcium hydroxide. By contrast, KLG productivity, yield, titer and cell viability are all significantly reduced when monovalent bases, such as sodium, potassium or ammonium hydroxides are used to control pH during fermentation.
Therefore, the preferred fermentation product has been calcium KLG, Ca(KLG)2. The conventional approach to acidification of calcium KLG has been to add sulfuric acid to equivalence to form KLG acid (HKLG) and calcium sulfate. Because calcium sulfate is only sparingly soluble it needs to be filtered off. Residual soluble calcium sulfate is removed by cation and anion exchange steps. While effective, this process generates large quantities of calcium sulfate waste that must be landfilled at considerable cost. It also entails a relatively high capital cost due to the need for cation and anion exchange and evaporation steps to remove large amounts of water from a relatively dilute fermentation broth.
Accordingly, there is a need for an improved more economic method for splitting polyvalent metal salts by electrodialysis technique which permits employing bipolar membranes wherein the formation of unwanted precipitates and the disposal problems associated therewith are avoided, as are the problems of fouling electrodialysis cell membranes, and a reduction in costly downtime.
It is therefore a principal object of the invention to provide novel and improved methods for splitting salts which avoid or reduce the problems associated with insoluble precipitates forming and downtime when performed using bipolar membrane electrodialysis. With the improved methods of the invention, it is now possible to employ electrodialysis techniques with bipolar membranes in splitting salts having polyvalent cations without the added disposal problems associated with solid by-products. These inventors found that the introduction of inexpensive, readily available acid to the chamber where insoluble hydroxide precipitates would otherwise form, the development and accumulation of such unwanted solids is avoided or inhibited. Instead of unwanted precipitates forming, useful acid and base values are produced without solids impeding cell operations.
In meeting the above principal object, a method is provided for inhibiting the formation of precipitates, such as polyvalent metal hydroxides in splitting salts by means of bipolar membrane electrodialysis by the steps, which comprise:
(i) providing an aqueous solution of a salt particularly one having a polyvalent inorganic cation and an organic anion;
(ii) separating the polyvalent inorganic cation from the aqueous solution by transporting across a cation exchange membrane into a base compartment, and
(iii)neutralizing insoluble polyvalent metal hydroxide formed from hydroxyl groups supplied across a bipolar membrane by introducing to the base compartment an acid to form a soluble polyvalent salt product with the polyvalent inorganic cation.
The introduction of an acid into the base compartment of an electrodialysis cell where insoluble hydroxides form neutralizes the hydroxide, or inhibits the formation of a solid precipitate.
It is yet a further object of the invention to provide a method for splitting salts by means of bipolar membrane electrodialysis which generates and concentrates multiple useful products, which comprises the steps of:
(i) providing an aqueous solution of a salt comprising cations and anions;
(ii) separating the cations from the aqueous solution by transporting across a cation exchange membrane;
(iii) converting the separated cations into the corresponding hydroxide by the introduction of hydroxyl groups supplied across a bipolar membrane;
(iv) forming a first useful product by acidifying the hydroxide;
(v) forming a second useful product either by:
(a) acidifying the anions remaining in the aqueous solution after separating the cations therefrom (step ii) by the addition of protons supplied across a bipolar membrane to form the corresponding free acid of the anion, or
(b) by separating the anions from the aqueous solution by transporting through an anion exchange membrane, and converting the separated anions to the corresponding free acid by the addition of protons supplied across a bipolar membrane.
The salt splitting electrodialysis methods of the invention are useful in recycle applications, such as in the recovery of organic acids from fermentation broths, like HKLG, or where the desired acid is present as a weak salt, e.g., calcium lactate. In the case of the latter, lactic acid may be recovered from calcium lactate by the salt splitting methods of this invention employing bipolar membrane electrodialysis. The introduction of acid from an external source according to the improved methods disclosed herein avoids insoluble metal hydroxide precipitates from accumulating and fouling the cell stack. It is the bipolar membranes in the cell stacks which dissociate water to form hydroxyl ions and protons on opposing sides of the membrane at low potential
The electrodialysis methods with bipolar membranes according to the invention may be conducted in multi-compartment cells having at least two compartments. In most instances, salt splitting is performed in electrodialysis cells have two and three compartment configurations. Typically, two compartment bipolar membrane electrodialysis methods are employed when salt splitting forms a weak acid or weak base. The bipolar membrane is paired with a cation exchange membrane when a weak acid is formed, or an anion exchange membrane when a weak base is formed. Because the weak acid or base formed is sufficiently undissociated it is not transported across ion exchange membranes to any great extent. Thus, in lactic acid production, calcium lactate is passed through the diluate compartment of a two compartment electrodialysis cell fitted with bipolar membranes and cation exchange membrane. Calcium ions are transported across the cation exchange membrane to a base compartment where calcium hydroxide formed from hydroxyl ions from the bipolar membrane is acidified by the introduction of acid to neutralize any metal hydroxides. Lactate remaining in the diluate compartment is acidified by protons from the bipolar membrane. The lactic acid thus formed is a weak acid and does not undergo electro-migration.
The present invention also contemplates the use of three compartment electrodialysis cells with bipolar membranes. They are especially useful when splitting a strong acid and a base. The diluate stream with salt present is disposed between a base compartment and an acid compartment. The base compartment is separated from the diluate compartment by a cation exchange membrane, and the acid compartment separated from the diluate compartment by an anion exchange membrane. These three compartment units are bounded on the ends by bipolar membranes, which supply hydroxide ions to the base compartment and protons to the acid compartment. Thus, a three compartment bipolar membrane electrodialysis stack consists of at least one of these three compartment units disposed between an anode and a cathode.
During calcium KLG salt splitting, for instance, the calcium migrates from the diluate compartment to the base compartment to react with hydroxyl groups supplied by the bipolar membrane. Instead of forming insoluble precipitate, acid introduced into the base compartment neutralizes the hydroxide to form a soluble salt of the neutralizing acid. Simultaneously, KLG anion from the diluate is transported across the anion exchange membrane to the acid compartment where protons supplied by the bipolar membrane provide the desired KLG free acid for conversion into ascorbic acid.